Biomedical polymers, hydrogels or gels are a relatively recent development which has had a substantial impact on the biomedical technology. These synthetic, polymeric materials have been used at the interface between various biomedical devices, e.g., electrocardiogram (ECG) electrodes, TENS electrodes, electrosurgery return pads, iontophoresis electrodes, ultrasound electrodes, wound dressings or coverings and skin or surface of an organ. They have largely supplanted naturally-occurring polymers such as karaya gum and guar which were the biomedical materials of choice before development of synethetic hydrogels. As has been described in the literature, synthetic biomedical polymers are a significant advance over their naturally-occurring counterparts in that the properties of the synthetic materials are more controllable, less expensive and more uniform. Given that these materials are used in medical and biomedical applications, the importance of controllability and uniformity of their properties is readily apparent.
Synthetic biopolymers, such as conductive, polymeric hydrogels or gels, do have one significant drawback in that upon storage, these materials tend to lose some of their water content (i.e., they tend to dehydrate or "dry out"). Since these materials normally conduct electricity city (and in some cases, sound), the loss of water tends to adversely effect a very important characteristic. Prior to this invention, there was no easy way for the user of a biomedical device having a polymeric hydrogel in a working surface or skin-contacting surface thereof to tell whether the hydrogel had been dehydrated to the point of significantly altering its properties (e.g., conductivity) short of applying the biomedical device to the patient and testing it. This invention provides an easy, visual method for detecting whether biomedical gels or hydrogels have lost an excessive amount of water so as to reduce their conductivity (to electricity or sound) and stability so as to not be suitable for the purpose intended.